GRAIL Mission Design and Timeline

Both GRAIL
Spacecraft will launch side by side on the same launch vehicle. The Delta II
vehicle has to launch within GRAIL’s launch window that extends from September
8 to October 19, 2011 to allow the mission to take place as it was designed.
When the spacecraft have been inserted into their desired trajectories, they
will separate from the launcher. Shortly after spacecraft separations, the
vehicle’s signals will be acquired by the Deep Space Network Station in
Goldstone, California. Initial activities in space will include the deployment
of the solar arrays and spacecraft health checks.

Source: NASA

Juno Launch Configuration - Two Spacecraft next to each other

The mission
was designed to aviod two lunar eclipses (at December 10 and June 4) because
the vehicles can’t pass through the dark for the duration of the eclipse. Both Spacecraft will take the slow road to the
Moon, taking up to two months. GRAIL will travel on the 'Sun-Earth Lagrange Point 1' (a gravitationally stable point in space, one of 5 Lagrangian points). This Earth–Moon L1 allows easy access to lunar orbits with smaller changes in velocity. Apollo Astronauts completed their journey to the Moon in three days. The
trajectory that was chosen for GRAIL is more energy efficient than the faster
alternative. It will also prompt the spacecraft to arrive at the Moon with much
less relative velocity, requiring a smaller engine firing to reduce their speed
enough to enter an orbit around the moon, so that less propellant is needed.
Another process that requires time is the so called outgassing of the spacecraft.
When a spacecraft enters space, small amounts of gas are vented from its
materials and structures. Those gases would interfer with precise gravity measurements in lunar orbit. During the
longer cruise time, all those gasses can be vented. To assist with outgassing,
the vehicles will be rotated so the sun can increase the rate of venting these
gasses. If GRAIL launches later in the launch window, more maneuvers will be
made to compensate the time that was lost, so that lunar orbit insertion (LOI)
will be on a fixed date.

Photo: NASA

A Delta II Launch Vehicle will put GRAIL into its trajectory to the Moon

Source: NASA

GRAIL's Trajectory to the Moon via the L1-Point between Sun and Earth

Source: NASA

GRAIL's Lunar Orbit has to be reshaped to accomplish Mission Objectives

GRAIL-A will perform its
lunar insertion around December 31, 2011. GRAIL-B will follow 25 hours later. This
interval has been inserted so that teams can focus on one spacecraft at a time.
In addition to that, Deep Space Network Antennas in Goldstone and Madrid
(Spain) will be having a good view of GRAIL-A performing the critical maneuver.
The DSN Stations will also have a good view of GRAIL-B when the Eath has
rotated once in between insertion maneuvers. The LOI maneuver will be 1 hour in
duration and place the spacecraft in a 8-hour capture orbit. The Lunar Orbit Insertion Burn will last 38 Minutes for each Spacecraft.

Source: NASA

The intial
lunar orbits of both vehicles will be different and highly elliptical. Orbits
have to be ciruclarized and aligned before science measurements will be taken.
Reshaping the orbits will take about 2 months and require about 20 maneuvers of
each vehicle. Both spacecraft will end up in a 113-minute, polar orbit that is
identical to the other GRAIL’s trajectory. The
mission’s science phase is scheduled to begin on March 8, 2012 to allow maximum
science time before the flight is terminated. 82-days of science orbits is the
maximum amount of actual data acquisition time that teams have determined.Each day, two 8-hour Deep Space Network Passes will be used to downlink all science and MoonKAM data/images. The science mission is devided into ten intervals, each lasting 27.3 days – a full revolution of the Moon beneath the orbiting spacecraft.

Orbital
Altitude of both vehicles will vary over time so that different aspects of the
Moon’s gravitational field can be measured. Natural conditions in the Moon’s
vicinity will ‘automatically’ change the orbital altitude of the spacecraft
from 15 to 50km and back down again.

Two small maneuvers are planned to adjust the distance between the Orbiters. The first one will be made at the start of the science phase and the second will be made when the first mapping cycle is complete and the vehicles mapped the entire Moon once. The distance will be changed to 225km apart from ~85km apart. Later, it goes back down to 65km again. After the science phase, a 5-day decommissioning period is scheduled that will feature several maneuvers to set the stage for both vehicles to impact the Moon in around 20 days.

Did you know?Two of the most common questions astronauts get is 1) 'How do I become an astronaut?' and 2) 'What type of salary, funding and benefits do you get in the business of space exploration?' To become an astronaut, you must meet basic requirements to be selected for astronaut training. The astronaut field is very competitive, with only about 20 openings every year. Nasa.gov has more information on the Astronaut Selection Office. As for the financial business associated with the salary, it varies depending on achievements and experience. Much like 401k business funding an astronaut's salary might increase over the years.

GRAIL Mission Timeline

Date

Event

Phase Duration

September 10 - 14:08

Launch

1 Day

September 11

Cruise Begins

108 Days

September 15

1st GRAIL-A TCM

--

September 16

1st GRAIL-B TCM

--

September 28

2nd GRAIL-A TCM

--

October 3

2nd GRAIL-B TCM

--

November 16

3rd GRAIL-A TCM

--

November 21

3rd GRAIL-B TCM

--

December 9

4th GRAIL-A TCM

--

December 14

4th GRAIL-B TCM

--

December 23

5th GRAIL-A TCM

--

December 24

5th GRAIL-B TCM

--

December 28

LOI Phase

6 Days

December 32

GRAIL-A Lunar Orbit Insertion

--

January 1

GRAIL-B Lunar Orbit Insertion

--

January 2

Orbit Adjust Phase

35 Days

January 10

GRAIL-A PRMs 1-4

3 Days

January 17

GRAIL-B PRMs 1-4

3 Days

January 24

GRAIL-A PRMs 5-12

3 Days

January 31

GRAIL-B PRMs 5-12

3 Days

February 6

Transition to Science Phase

32 Days

February 7

1st GRAIL-A TSM

--

February 14

1st GRAIL-B TSM

--

February 20

2nd GRAIL-A TSM

--

February 22

2nd GRAIL-B TSM

--

February 24

3rd GRAIL-B TSM

--

February 28

4th GRAIL-B TSM

--

March 7

Science Phase Begins

82 Days

March 7 - 1:15

Mapping Cycle 1

27.3 Days

March 28

GRAIL-A Orbit Trim Maneuver 1

--

April 2

GRAIL-A Orbit Trim Maneuver 2

--

April 3 - 9:15

Mapping Cycle 2

27.3 Days

April 30

GRAIL-B Orbit Trim Maneuver 1

--

April 30 - 17:15

Mapping Cycle 3

27.3 Days

May 5

GRAIL-B Orbit Trim Maneuver 2

May 28 - 1:15

END PRIMARY MISSION

May 29

Eclipse Set-Up

June 4 - 11:03

Lunar Eclipse -- Duration: 128min

128 Minutes

August 30

START EXTENDED MISSION

August 30

Mapping Cycle 4

27.3 Days

September 26 - 8:00

Mapping Cycle 5

27.3 Days

October 23 - 16:00

Mapping Cycle 6

27.3 Days

November 20 - 0:00

Decommissioning Phase

5 Days

November 25

End Decommissioning Phase

~20 Days

November 25

End of Mission Operations

December 2012

Moon Impact - EOM

--

Mission Phase

2011

Mission Phase

2012

Mission Phase

2013

Mission Milestone

TCM

Trajectory Control Maneuver

Dates are subject to change

PRM

Period Reduction Maneuver

Dates are subject to change

All Times/Dates GMT

***End of Mission Dates based on estimated values.***

Launch Phase This Phase begins prior to liftoff when the Spacecraft are switched to internal power during the final minute of the last built-in countdown hold at T-4 Minutes. The rest of the phase obviously includes launch, powered flight, solar array deployment and initial spacecraft status polls and checkouts. 24 hours after launch, the first Mission Phase Transition is being performed. GRAIL has a 42-day launch period between September 8 and October 19, 2011. On each day, two instantaneous launch opportunities are available to the launch team. The first one puts GRAIL into a 93-degree flight azimuth and the second targets a 99-degree azimuth.

At the time of liftoff, six of the SRBs are ignited and the Delta II 7920H-10 rocket soars into the sky. After 74 seconds, the SRBs are burning out and the remainig three are ignited. Following first stage cutoff and stage separation, the second stage performs its first engine burn to put GRAIL into a Low Earth Parking Orbit. The vehicle will coast for nearly an hour before the second stage engine fires one more time to put the twin spacecraft into their Trans-Lunar Trajectory. Three minutes after the final burn, spacecraft separation begins. The first step of that is the reorientation of the stack to the proper separation attitude. At Cutoff+9.5 Minutes, GRAIL-A is being separated by cutting breakwires. The second stage will perform another attitude adjust maneuver prior to GRAIL-B separation. 8 Minutes and 15 Seconds after GRAIL-A Sep, GRAIL-B will be released. Separation occurs in sunlight and tracking will be available via the Goldstone DSN Antenna that will pick up spacecraft signal within one minute following Sep.

Solar Array Deployment will be just minutes later. The arrays on each of the vehicles will deploy within 2 Minutes and 7 Seconds after initiation of the deployment. Telementry will immediately show a successful deployment which is a critical mission objective.

Source: NASA

Trans Lunar Cruise 24 Hours after launch, the vehicles will transition to this phase. Both GRAIL orbiters will be on a low energy trajectory to the Moon via Earth-Sun Lagrange Point 1. During this journey, more vehicle checks will be performed. Up to five Trajectory Correction Maneuvers can be made during this phase. Two of those maneuvers will separate GRAIL-A and B in order to correctly time their respective arrivals at the Moon with an interval of about one day.

Source: NASA

Lunar orbit insertion LOI begins three days before GRAIL-A arrives at the Moon. It is primarily focused on the critical maneuver that puts the spacecraft into lunar orbit. Arriving over the lunar South Pole, GRAIL will fire its engine for 38 Minutes to change its velocity by 427 miles per hour. GRAIL-B will make an identical maneuver only 25 hour later. The timing of those maneuvers has been set very carefully. Two Deep Space Network Stations (Goldstone and Madrid) will have contact during the LOI Burn so that redundant communication paths can be established.

Orbit Reduction Period One day after LOI, this phase begins. About 20 Maneuvers for each orbiter (devided into two clusters) will be used to reduce the orbital period of GRAIL from the initial 11.5 hours per orbit to 113 minutes. Also important is that both spacecraft end up in an identical orbit chasing each other around the Moon when this phase has finished. Otherwise, science data gathering would not be possible.

Transition to Science Phase From launch to a point in February of 2012, GRAIL-A and GRAIL-B are treated as individual spacecraft. The orbiters will have had separate trajectories, maneuvers and communication assets up to this point in the mission. In order to begin the science phase, the position of each vehicle relative to the other has to be calculated and precisely adjusted. At the start of this phase, GRAIL-B’s orbit will last three minutes longer than GRAIL-A’s meaning that B is flying higher than A. This situation enables engineers to perform maneuvers in a timed manner to organize the spacecraft correctly. A complicated series of rendezvous maneuvers will be flown to reach the intial separation distance with GRAIL-B leading the ‘convoy’.

Science Phase Targeted to begin on March 8, 2012, the science phase will gather all scientific data of this mission. During the 82-day science period, three mapping cycles will be performed. A mapping cycle is the time it takes the Moon to rotate one full revolution beneath the orbiting GRAIL vehicles. This time is 27.3 days in duration. Once the science phase starts, orbital mechanics will control the range between the spacecraft to a certain point. Mapping cycle 1 sees the spacecraft flying at a range of 100 to 225 kilometers. A minor maneuver close to the end of MC1 will adjust the drift rate between the vehicles for the next two cycles. The range will then decrease to 40 miles at the end of the science mission. Close range lets the spacecraft detect local gravity elements while a bigger distance shows a more global gravity picture to the vehicles.

Extended MissionAfter the planned end of the Science Mission Phase, an extended Science Phase of 7 Mapping Cycles will follow. During that period, nominal science operations continue in order to obtain more garvitational data. GRAIL-A and B will stay in formation and continually lower their orbits coming as close as 7 Kilometers to the lunar surface. This altitude enables the instruments to detect even minute mass features of a smal ground swath of the Moon. Mean altitude will decrease to about 22.5 Kilometers.Mapping Cycle 4 is reserved for the Lunar Eclipse of June 4, 2012. The partial eclipse has a duartion of 128 Minutes during which the two Spacecraft will not receive any sunlight. Starting late in May, the vehicles are being configured for the event by powering down equipment that is not essential to maintaining spacecraft operations - for example the MoonKAM Payload. It is expected that the batteries of the vehicles will provide a constant power supply during the eclipse and that both, GRAIL-A and B, will survive the event. Should one or both spacecraft not make it through the eclipse in operating condition, the mission ends as both spacecraft are required for measurements of the lunar gravity field. Later in MC4, GRAIL Science Operations are restored and data acquisition resumes. In total, GRAIL will make 9 or 10 Mapping cycles.Decommissioning Phase When the science phase has ended, the final mission phase will begin. During a 5- to 7-day period, a Ka-Band calibration is made and GRAIL continue to provide science results as sunlight and power allows. Early in December, 2012, the GRAIL Mission ends. About 20 days after the end of this phase, the GRAIL Orbiters will impact the lunar surface because their orbits will not have been maintained. No special area has been targeted for impact.

GRAIL Distances Traveled

Launch to LOI

Lunar Orbit

Total

GRAIL-A

2,594,378 Miles

13,193,550 Miles

15,787,928 Miles

GRAIL-B

2,663,793 Miles

12,800,869 Miles

15,464,721 Miles

Distances do *not* include the extended mission.

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